Re: Angular momentum

  a  Moment of any physical quantity is it's rotational equivalent.  a
moment, therefore, is always relative to a centre or axis of rotation. it
is the product of that quantity and the distance from where the quantity
exists to the centre or axis about which you wish to evaluate the moment.
if the physical quantity is a vector, you must take a cross product.

for example, let us take force as the physical quantity . the moment of a
force about any point or axis  is R x F, where R is the distance from the
point of application of the force to the point or axis. since force is a
vector quantity, the x denotes a cross product. all a cross product does is
that it takes the component of the force perpendicular to the radius and
multiplies it with the radius. if the force were already perpendicular to
the radius, the moment of the force about the point would be simply the 
product RF. however, since the force is a vector, it's moment is also a
vector. the direction of it's moment is given by the cross product.

it might make more sense to you now if we said that angular momentum is
also sometimes referred to as Moment of momentum. you know that momentum is
a vector quantity, whose magnitude is the product of mass and velocity and
direction the same as that of the velocity vector. therefore it's moment
about a point or axis would be as before, R x P, where P denotes the linear
momentum vector. it's direction would then be perpendicular to the plane
containing R and P and it's sense given by the right hand rule.


your second question takes us to the realm of quantum physics. it is
postulated there that the angular momenta of the orbiting particles can
only take on values that are integral multiples of h/2*pi . you have to
look up a book on qm to get a proof.